In steam turbines a flow channel is formed by different delimiting parts or flow channel parts which are joined together. The flow channel formed is subjected to hot steam under high pressure. Along the longitudinal axis of the flow channel pressure chambers are formed with differing levels of internal pressure. It is thus of decisive importance for the connection between the different delimiting parts to be sufficiently tight so that no leakages occur. This is especially difficult since, with a steam turbine, steam temperatures or live steam temperatures of more than 600° C. can occur at a steam pressure of more than 250 bar.
At the connection between at least two delimiting parts of the flow channel a joint is formed, with four delimiting parts a so-called cross joint. A cross joint features both a horizontal joint and also a vertical joint. It is necessary to form a cross joint if, for manufacturing reasons or because of the materials selected, the first delimiting parts and the second delimiting parts of the flow channel must be embodied separately. As a rule, the delimiting parts feature different materials with different coefficients of thermal expansion and different constructions. Thus one delimiting part can be formed from a steel casting and the other delimiting part can be formed as a welded construction or from a spheroidal casting. The flange connection at the joints, especially the cross joints, is to be designed for overpressure, for a vacuum or mostly for changing pressure conditions.
The design of steam turbine housings with a cross joint has the advantage of the manufacturing of the blanks and the processing of the delimiting parts being able to be undertaken on smaller components in each case. This allows benefits to be obtained both in relation to the costs and also in relation to procurement and processing capacity. The disadvantage of cross joints however is that, because of the limited space around the intersection point of the joints, there are only limited opportunities for screwing them securely together. Thus there is a greater danger of leakage with cross joints than with other joints and they are only designed for steam turbines up to specific restricted steam parameters.
The practice of equipping cross joints with suction outlets is known in order to catch any leakage quantities occurring. Also known is the practice of welding a sealing seam onto the inside of the vertical joint. The problem here is that of designing the welded joint to be secure and allow sufficient thermal expansion, so that damage is avoided even with non-steady-state processes. The high pressures and temperatures involved can result in expansions of the corresponding delimiting parts, so that cracks and thereby leakage can occur at the welded seam.